Green algae on external walls: causes, fixes and what surveyors look for

Green algae on walls is easy to overlook - but a surveyor won't miss it, and neither should you.

A well-maintained garden is a genuine pleasure. Fresh air, green space and a sense of nature on your doorstep are all well-documented benefits - which is why the idea of escaping to the countryside holds such appeal for so many people.

But there is greenery that nobody wants. We have covered problem plants before - Japanese knotweed being one of the more alarming examples - but what about that other unwelcome growth: the green algae or mould that creeps across external walls, timber frames, plastic fascias and roof surfaces?

It is more than an eyesore. Left unmanaged, it is a flag that something is wrong with the property and a chartered surveyor will treat it as exactly that.

Like black mould inside a property, algae thrives where conditions are persistently damp. That typically means areas that receive little or no direct sunlight and have poor airflow, so surfaces never fully dry out. In the UK's climate, with significant rainfall throughout much of the year, these conditions are far from uncommon.

Blocked or leaking gutters are a particularly common cause. Where rainwater is not being properly directed away from the building, it runs freely down the walls and saturates the surface. Over time this can also drive damp inside the property, creating the conditions for black mould growth internally.

In winter, poorly insulated walls create cold spots where condensation forms and that;s another environment in which algae can take hold.

The problem is compounded by the fact that most construction materials: brick, render, concrete, timber are porous to some degree. They hold moisture and, in doing so, provide a ready surface for biological growth.

Left untreated, algae does not just look bad. It accelerates the deterioration of the materials it grows on, working its way into the surface and making the underlying problem progressively worse.

For a chartered surveyor, visible algae growth is clear evidence that warrants closer investigation to establish the root cause, assess any damage to the fabric of the building and identify what remedial action is needed.

For existing growth, the approach is fairly straightforward. A stiff brush can remove loose growth, and a pressure washer will shift the worst of it. White wine vinegar applied to the surface is an effective traditional remedy, and biocidal cleaners are also available though these should be used with care around other plants and wildlife in the vicinity.

There are also preventative spray products designed for walls, roofs and other exposed surfaces that inhibit algae from taking hold in the first place. None of these require specialist application.

For buyers, first impressions form the moment you pull up outside a property. Evidence of poor maintenance like rotting window frames, a neglected garde or dilapidated outbuildings registers before you have even stepped through the door. Algae growth on external walls falls into the same category. If the current owners have not addressed something as straightforward as green mould on the front elevation, it is reasonable to wonder what else has been left unattended.

For sellers, it is a simple fix that costs relatively little in time or money but can meaningfully affect how a buyer perceives the property from the outset. Sorting it before going to market is a sensible step.

For buyers who want certainty about everything behind the surface and not just what they can see, a survey from a RICS-accredited chartered surveyor will report on all identified issues, their severity, the recommended remedial action and an estimate of cost.

Surveyor Local connects you with a nationwide panel of RICS-accredited chartered surveyors who will assess the full condition of a property before you commit to buying.

Call us on for a no-obligation quote, or use our online quote generator to get an instant price.